Osteoporosis is a complex disease, from a genetics standpoint. Bone mass and structure are highly heritable traits, but there are probably many genes that contribute to these traits. We have identified quantitative trait loci (QTL) in B6C3F2 mice that are linked to bone structure and strength. In addition, we have developed 11 congenic mouse lines, each of which contains a bone structure/strength QTL. Our preliminary studies demonstrate that the progenitor mouse strains (C57BL/6 and C3H/He) differ considerably in their skeletal response to mechanical loading. Furthermore, we demonstrated in a congenic mouse line, B6.C3H-4T, a significantly enhanced skeletal responsiveness to mechanical loading. Not surprisingly, B6.C3H-4T femurs have significantly larger cross-sectional size compared to B6 control femurs. This finding suggests that congenic mice can be used to identify genes that affect cellular mechanotransduction in bone. We have identified four congenic mouse lines (in addition to B6.C3H-4T) that have altered femoral crosssectional size. We propose to determine which of these congenic lines differ in skeletal mechanical loading response. We will then develop congenic sublines and complete fine mapping of the QTLs to isolate genes contributing to altered mechanotransduction. We will develop multiple sublines of congenic mice to genetically dissect each QTL region to better pinpoint the location on the chromosome contributing to femoral BMD and/or structure/strength phenotypes. In addition we will study osteoblasts isolated from congenic mouse lines to determine their responsiveness to mechanical stimuli and we will examine gene expression profiles in bones from congenic mice and in isolated osteoblasts in order to determine the genetic pathways that differ among the congenic lines.